You’ve probably heard astronauts talking to mission control while they perform operations in space. In these recordings, you can hear the back-and-forth chatter, along with the astronaut’s breathing and the background noise of their spacesuit pumping oxygen into their helmet to keep them alive. Yet, if they removed that helmet and broke the barrier of the suit shielding them from outer space, that conversation would be cut — and all sound would go radio silent.
As astrophysicist Neil DeGrasse Tyson once explained on the podcast StarTalk, astronauts would be able to hear things from within the body itself — like their own heartbeat.
“The sound of silence is the sound of things that were always making noise that you never noticed before,” he said on the podcast.
Sound waves are a vibration carried through some sort of medium, like air or water or in the case of the heartbeat, the body. When those vibrations reach our ears, they send a vibration through our eardrums, which is recognized in the brain as sound.
Because sound needs something to travel through, it can’t make its way through the vast majority of space, which is a vacuum containing essentially no particles. Interplanetary space contains just a few dozen particles in each cubic centimeter — in comparison, the air we breathe has tens of quintillions of molecules per cubic centimeter. (For scale, 10 quintillion seconds is longer than the age of the universe.)
“In the universe, an absolute vacuum is rare, and most of the universe is very low-density high-temperature plasma,” said Chris Impey, an astronomer at the University of Arizona. “In principle, sound could travel through that, but it would have very different properties to what we are used to.”
"In the universe, an absolute vacuum is rare, and most of the universe is very low-density high-temperature plasma."
Gas clouds, dust clouds and solar winds for example, could all have sound waves pass through them, even if they are relatively low-density, said Phil Plait, an astronomer who runs The Bad Astronomy blog. The structures of many gas clouds, for example, can be formed by sound waves, or shock waves in the case that the material moves faster than the speed of sound, he explained.
“We see the effects of sound in these objects all the time,” Plait told Salon in an email.
This would be nothing like the sound we are used to on Earth and wouldn’t be detectable by the human ear, which can only hear a very narrow range of frequencies. You may remember the black hole in the Perseus galaxy cluster about 250 million light-years away, from which NASA detected emanating pressure waves in 2003.
Although this was not a sound recording like you would hear from a microphone, NASA did convert these pressure waves into sound, albeit one that is far too low of a frequency for the human ear to detect. For what it’s worth, though, they did find that the waves corresponded to the note of B-flat, about 57 octaves below the middle C note on a piano.
Then, in 2022, NASA’s Chandra X-ray Observatory sonified this wave data into a couple of sounds the human ear could hear at frequencies 144 quadrillion and 288 quadrillion times higher than the original. (To get a sense of just how astronomical this figure is, one study estimated that there are 20 quadrillion total ants on Earth.)
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"What's going on is that matter is surrounding the black hole, and when some stuff falls in it can create a powerful wind that compresses the material around it, making a sound wave,” Plait said. “We don't detect the sound itself, but we can see the ripples in the gas and they can be converted into sound we hear.”
There are entire projects dedicated to sonifying data from astronomical objects. In the Cassini mission, for example, NASA detected radio waves emitted from charged particles in magnetic fields, which were converted to sound. Still, these were plasma waves, and not sound waves.
However, sound has been detected within our own solar system. During NASA’s Perseverance mission on Mars in 2021, the rover’s microphones detected the whir of the mission’s helicopter and noises created by the rover. It also detected naturally occurring sounds on the planet itself — including Martian wind. Back in 1981, Russia also reported sounds on Venus during the Soviet Venera 13 mission, which sounds like waves hissing on a beach.
Yet sounds on other planets sound different than they do on Earth because other planets have different atmospheres. On Earth, the unique combination of oxygen, nitrogen and other gases, combined with the effects of gravity and solar heating, create a certain density of molecules that carries sound as we know it.
In contrast, the atmosphere on Mars is roughly 2% as dense as Earth’s, and its composition is dominated by carbon dioxide. Overall, sounds would be quieter and slightly muffled, and it would also take longer to reach you than it would on Earth. Some higher pitched sounds would be inaudible entirely.
Interestingly, if you played a church organ on Mars, the set of flue pipes that create sound in a way similar to a flute would go up in pitch, but the reed pipes, which produce sound in a way similar to a saxophone, would go down in pitch, said Tim Leighton, an acoustics professor at University of Southampton, who created models to predict sound on other planets.
The first million years of the universe … sounds like "a descending scream, a deep roar and a final growing hiss."
Saturn’s moon, Titan, is probably acoustically the closest to Earth. However, the pressure and density are a bit higher at ground level, and the speed at which sound travels through the atmosphere is lower than Earth. As a result, many sounds such as voices, flutes and organ pipes would play at a lower pitch, Leighton said.
On Venus, sounds that are caused by solid objects vibrating, like harmonicas or reed organ pipes, would be pitched down because the atmosphere is dense and soupy. However, sounds from things like flue organ pipes or flutes, which are propagated through air, would be pitched higher than Earth. That’s because the extremely hot temperatures on Venus make sound travel faster than on Earth.
Additionally, if we theoretically heard a sound like a vocalization on Venus, our perception of the size of the creature it was coming from would be a little distorted. That's because humans evolutionarily developed a way of hearing vocalizations in which sound travels to the top of the nose of the speaker and back again in a form of echo, which we subconsciously use to estimate how large a creature is based on the tone they emit, Leighton said.
On Venus, "this pulse quickly travels up to the top of the nose and back again much sooner than it would on Earth,” Leighton told Salon in a video call. “Your brain hears that and imagines the person is about three feet tall.”
As we continue exploring more distant planets, recording sound could help scientists better understand them. For example, measuring the sounds of wind on Mars could provide clues on how the planet’s surface forms, Leighton explained.
“It can tell us a lot about the atmosphere and how it changes as the sun goes up and down, and how that, in turn generates winds to shape the surface of Mars,” Leighton said. “That indicates the power of these microphones.”
Sound could also help us explore planets like Jupiter and Saturn, which likely have plenty of sound to hear but have thick clouds and inhospitable conditions that make it difficult to access visually, Impey said.
“In fact, since the atmosphere is sort of opaque and you can't really see through it, it might be a way to sense what's happening better and more efficiently than you could with any sort of a camera, which wouldn't really work very well at all,” he told Salon in a phone interview.
When looking for sound in the universe, astronomers have also looked back in time. Back in the early years of the universe, it was a hot plasma soup that was far more dense. That plasma carried acoustic oscillations, although still not at an audible range. However, in one research project, astronomer Mark Whittle compressed the first million years of the universe into 10 seconds, shifted up by 50 octaves so that the human ear could hear. It sounds like "a descending scream, a deep roar and a final growing hiss," he reported.
About 400,000 years after the Big Bang, sound waves called Baryon acoustic oscillations rippled through the cosmos to influence how galaxies were distributed. As such, one could say that life on Earth as we know it in some way originated from a sound wave. It’s not called the Big Bang for nothing, after all.
“Within that sea of brilliance, the seeds for all that we now know were already present, latent, waiting to unfold,” Whittle wrote in his report. “Most remarkable of all, perhaps, these seeds were sounds – pressure waves coursing through the fluid.”
